The present invention is directed to a variable resistance lifting mechanism and, more particularly, to a mechanism for a machine which provides the proper increased resistance during an exercise stroke to maintain maximum muscular effort throughout the range of movement of the user.
There are natural changes which occur in the human lever system while performing movements that necessitate different levels of muscular involvement. The variation in muscular force or involvement results from the biomechanical advantages and disadvantages of the human lever system. When the human lever is in the position of greatest biomechanical disadvantage, maximum muscular involvement is required. When the human lever system is at its greatest advantage, minimum muscular force is required.
In order to provide the desired constant maximum muscular involvement throughout an exercise stroke, it is necessary to vary the resistance so that when the human lever is in a position of greatest biomechanical disadvantage, the resistance is at a minimum. Correspondingly, the resistance should be increased as the biomechanical advantage of the human lever system increases. Thus, by properly varying the resistance of the exercise machine during the exercise stroke, it is possible to maintain the same degree of muscular involvement throughout the entire exercise stroke.
Various prior art devices have sought to provide variable resistance during an exercise stroke. The most widely used prior art device includes a cam wheel pivotally mounted to the frame of an exercise machine. This device includes a chain secured tangentially to the cam wheel at one end, wrapped around a roller, and having weights secured to the opposite end of the chain. A lever arm is connected to the cam wheel and a user exerts force aginst the lever arm to rotate the cam. As the came rotates, the distance from the cam pivot point to the point on the cam surface from which the chain extends tangentially increases. This increases the moment arm of the cam wheel and correspondingly increases the force a user must exert against the lever arm to rotate the cam wheel.
Such cam wheel devices are extremely heavy and require numerous mechanical components which create excessively high moments of inertia. This adverse inertia force becomes the dominant driving force, particularly when the exercise stroke is performed at a rapid speed, and therefore effectively distorts the pattern of increased resistance and effectively prevents constant maximum muscular involvement during the exercise stroke.
In order to avoid the inertia problem of cam wheel devices, a second type of prior art variable resistance exercise device was designed which includes a lever arm pivotally attached to a frame having guide bars. Three rollers are secured to a U-shaped member with two guide rollers adapted to roll directly on the vertical guide bars. The third main roller is adapted to roll on the upper surface of the lever arm passing beneath the third roller and through the U-shaped member. The U-shaped member is rigidly connected to a selector bar which is in turn adapted to be attached to weights which slide on the guide bars. In the operation of such a device, as the user exerts force against the lever arm during the exercise stroke, the lever arm exerts force against the third main roller to lift the U-shaped member and the attached weights. The two guide rollers roll vertically upward on the guide bars to provide linear movement of the weights.
This prior art roller mechanism has two primary disadvantages. The first disadvantage is the tendency of the rollers to wear, thus increasing friction and requiring constant maintenance. The wearing of the two guide rollers prevents them from providing a relatively frictionless surface against the vertical guide bars. The wearing of the third main roller distorts the relatively frictionless surface against the lever arm.
The second disadvantage of the prior art roller mechanism is its creation of adverse bending moments against the vertical guide bars. The two guide rollers cause a localized tangential force against the guide bars at the point of contact the guide rollers with the guide bars which eventually tends to bend the guide bars. Furthermore, the tangential force of the third main roller aginst the lever arm causes a localized bending force on the lever arm. These bending forces distort the pattern of increased resistance during the exercise stroke thus destroying the object of the device which is to maintain constant maximum muscular involvement throughout the exercise stroke.